Original radiator pressure cap for MGA has 1-1/8-inch reach to fit 1-inch deep filler neck. I like to call this the "long reach cap" for the "deep filler neck". The MGB and other later model cars commonly have a shorter filler neck requiring a 7/8-inch reach cap for 3/4-inch deep filler neck. Please do not confuse these different configurations. The long reach cap cannot be installed in the shallow (3/4" deep) filler neck. If you install a short reach cap in the deeper (1" deep) filler neck it will not hold pressure, same as having no cap at all.

Please review article CO-101 on Keeping your cool", and article CO-204 on Coolant recovery system". Early production MGA pressure cap had 4-psi relief pressure rating. In July 1959 at Car Number 71832 this was changed to 7-psi, and it is highly recommended that earlier cars should be retrofit with the 7-psi cap. This should not deduct points in a concours car show, as it is to be considered original by the factory design change and Confidential Service Memorandum CSM_MG/289 noting the upgrade to earlier cars.

At the inside top of the pressure cap there is a wide flange, actually a springy diaphragm, that covers the top of the filler neck. This generally prevents fluid from spilling over the top of the neck and directs the fluid to flow out through a small pipe on the side of the filler neck. That pipe is originally routed to a point low on the side of the radiator where any escaping fluid can drain on the ground. The diaphragm in the top of the pressure cap does not (originally) have to be absolutely pressure tight, just good enough to direct most of the overflow into the drain tube, and not allow much fluid to dribble out the top of the filler neck. As such, the original cap design used a bare metal diaphragm with metal to metal contact between the diaphragm and the top of the filler neck.

For the purpose of installing the coolant recovery system, the pressure cap DOES need to have an absolute seal at the top of the filler neck, because it has to be able to create a slight vacuum to draw fluid back into the radiator when the system cools down. You will notice in the picture above that the top diaphragm in the cap has a thin rubber seal around the edge. This is not included on the original MGA radiator cap. It was not used much prior to 1968 (when radiators had shorter necks), so the demand is only for aftermarket conversions on older cars.

The radiator cap in this picture is the long reach cap with the top seal, one of those very rare items. This one is NAPA-Balkamp part number 703-1411, but ordering this part number does not guarantee that you will get what you need. In past years this same part number was used for the original type cap with no rubber seal at the top. Around 2001 or (thereabout) the cap design was changed to include the top seal. Since the new design works equally well in older applications, the new part supersedes the old one, and the same part number is still used. With demand being so low, and sales slow, it is possible that there may still be some of the older parts still in stock on the shelves for years to come. As recently as summer 2003 my local NAPA dealer had an older part on the shelf in Illinois, while a friend found this newer one for me at a NAPA dealer in North Carolina. So if you're buying one of these parts, open the box and check it before you walk out of the store.

Pressure caps, 7-psi with 1-inch reach, as pictured above that will work for coolant recovery:

Manufacturer

Part No.

NAPA/Balkamp

703-1411

Stant

10206

Cross reference numbers for the STANT 10206 pressure cap. Check to be sure if these will or will not have the top seal to work for coolant recovery.